High-frequency alternating-current amplifier



Patented Get. 18, 1927.

UMTED snares 1,646,364 PATENT GFFICE.

MILLARID COLE SPENCER, OF EAST ORANGE NEW JERSEY, ASSIGNOR TO CROCKER- WHEELER ELECTRIC MANUFACTURING COMPANY, OF AMPERE, NEW JERSEY, A.

' CORPORATION OF NEW JERSEY.

HIGH- FREQUENCY ALTERNATING-CURRENT AMPLIFIER.

Application filed March 3 1924. Serial No. 696,671.

This invention relates to improvements in amplifiers for radio-frequency currents using three-element vacuum tube, or audion type amplifiers.

The object of this invention is to produce a simple method or means whereby an amplifier, under such conditions, can be kept from oscillating owing to the capacity coupling between the grids and the plates of the tubes and the magnetic coupling between the amplifier transformers. Owing to the structure ot a three-element vacuum tube, the grid has a certain electrostatic capacity with respect to the filament and with respect to the plate which cannot be avoided. This electrostatic capacity between the grid and the filament provides for. currents of the frequencies here considered, a relatively low impedance path for such currents fromthe' grid to the filament and prevents the grid voltage from being built up asdesired.

This di'liiculty can be overcome by tuning the grid circuit. By this means the eiiective impedance between the grid and the filament may be made as high as desired. However, the tunin of the grid circuit introduces another difiicu'lty caused by the capacity between the grid and the plate of the amplifier tube. This capacity coupling between the grid and the plate is sufficient, when the grid is tuned, to cause the tube to oscillate when the effective reaction in the plate circuit is inductive and equal to a certain critical value. A small transformer is ordinarily used to transfer the high-frecircuit of the tube to the grid circuit of the the next tube. When transformers of the. construction usually employed for this purpose are used with a tuned grid circuit, then the effective reaction introduced into the plate circuit may, and, at certain adjustments of the tuned grid circuit, will be inductive and of suflicient value to cause the tube to oscillate. When a tube does so oscillate then its own oscillations may be magnified to such a high degree as to interfere with, or entirely prevent, the desired amplifying action of the tube and cause the action of the whole amplifier to become unstable. One of the objects of this invention, then, is to so proportion the constants of the transformers used to transfer the alternating currents from one tube to the next,

'erly. It is only .over a narrow vented.

and in particular the magnetic coupling between the primary and the secondary windmgs, so that at no adjustment, within the range of adjustments provided for the tuned remainsto enable the stage to function proprange of coupling that this holds true. If the cou pling is too small then the desired amplifying action is reduced or even entirely pre- Aeecond object of this invention is to provide means for preventing magnetic couphng between the amplifier transformers. Th1s ob ect is attained by placing the transformers with their centers in line and their axes mutually at right angles to each other.

In the accompanying sheet of drawings which forms a part of this specification- Figure 1 is a plan view of an amplifier embodying this invention which shows the physical arrangement of the several elements, with magnetic coupling. quency alternating current from the plate Fig. 2 is a diagram of the connections.

Fig. 3-is a diagram of the connections in a modification in which capacity coupling 1s used instead of magneticcoupling.

In the form shown in Figs. 1 and 2, in ut terminals 9 and 10 receive the highrequency current which is to be amplified. They are connected to the primary winding 11 of an input transformer. ondary winding 12 of this transformer is connected to the. grid and filament of a first stage threeelement amplifier tube 13. The plate of this tube is connected through the primary winding 14 of a first coupling transformer, and the secondary 15 of this transformer is connected to the grid and filament of a second stage three-element amplifier tube 16. The plate of this second stage tube is connected through the primary winding The sec- 17 of a second coupling transformer, and the secondary winding 18 of this transformer is connected to the output terminals 19, 20. The filaments of the amplifier tubes are connected to an A battery 21 the current of which is regulated by a rheostat 22, and the primary windings of the first and second coupling transformers are connected to a B battery 23 which is a source of high voltage direct current Provision is made for tuning the grid circuit of the first stage amplifier tube by means of a variable condenser 24which is connected across the terminals of the secondary of the input transformer. The grid circuitof the second stage amplifier tube is tuned by means of a ,variable condenser 25 which is connected across the terminals of the secondary of the first coupling transformer. The output circuit is tuned by: means of a variable condenser 26 which is connected across the terminals of the second coupling transformer.

The primary and secondary coils of each transformer have the same center, and the three transformers are physically located with their centers in line and their axes mutually at right angles to each other. Magnetic coupling between the transformers is in this way prevented and the tendency for the tubes to oscillate due to magnetic coupling is thus avoided.

The grid circuit of tuned to resonance at the frequency of the alternating current which it is desired to amplify, by means of the secondary coil of the input transformer and its associated variable condenser. In this way the detrimental capacity effect between the grid and filament of the first stage amplifier tube is eliminated. In the same Way the grid circuit of the second stagev amplifier tube is tuned by means of the secondary coil of the first coupling transformer and its associated condenser and the detrimental capacity effect between the grid and filament-of the second stage amplifier tube is eliminated.

WVhen the grid circuit of the first stage amplifier tube is tuned to resonance at a given frequency and the effective reaction in the plate circuit for this same frequency is reactive and equal to a certain critical value, then the tube will oscillate due to the inherent capacity coupling between the grid and plate of the amplifier tube. Such undesired oscillations are prevented'by making the inductive reaction of the primary winding of the first coupling transformer too small to cause the tube to oscillate, for example by using only three to ten turns and also by making the coupling between the primary and the secondary coils sufficiently small, pre terably below forty percent, which is effected, for example. when there are only three to ten turns in the primary Winding,

by making one coil, preferably the primary the first stage tube is vcoil, about two thirds the diameter of the other coil of the transformer. Then the effective inductive reaction of the tuned secondary circuit back into the. primary circuit for any adjustment of the condenser in the secondary circuit will be too small to cause the first amplifier tube to oscillate. This reduction in coupling can be made and still leave suilicient coupling to enable the stage to function properly This particular value of coupling can best be found by trial and when once obtained will be found to hold over quite a wide range of adjustments ofthe condenser and need not be changed. In the same way the coupling between the primary and tlie secondary coils of the second coupling transformer is adjusted by trial. The coupling between the transformer coils should be as great as possible without causing the tubes to oscillate in order thatthe amplifying'action of the tubes may be a maximum.

It is not essential to this invention to employ transformers with magnetic coupling Q to couple together the amplifier tubes,- and in Fig. 3 a modification is'shown in which the amplifier tubes are coupled together by means of resistances 30, 31 and series con-- densers 32, 33, so that the inductive reactions of the tuned circuit betweenthe filament and grid of the second amplifier tubeare kept below the value which would cause the first amplifier tube to oscillate by making the series condenser between the two amplifier tubes su'fiiciently small, and the inductive reactions of the'tuned output circuit are kept below the value which would cause the second amplifier tube to oscillate by making the series condenser between the outputcircuit and the plate of the second'amplifier tube sufficiently small. due to magnetic coupling between the coils are prevented by locating them with their centers in line and their axes at right angles to each other.

I claim Oscillations v 1. An amplifier for high frequency alter- I nating currents comprising three-element amplifier tubcs with tuned grid circuits in which the consecutive tubes are coupled together by transformers having primary windings with sufficiently small inductance and with sufficiently small coupling between the primary and the secondary windin s so that the effective inductive reaction 0 grid circuit of a tube in the plate circuit of the preceding tube together withthe inductive reaction of the plate circuit itself will always be below the critical value required to cause the preceding tube tooscillate, and means for preventing undesired magnetic coupling between the transformers.

2. An amplifier for high frequency alternating currents comprising three-element amplifier tubes with tuned grid circuits in which the consecutive tubes are coupled together by transformers having primary windings of approximately three to ten turns and two-thirds the diameter of the secondary Winding so that the efiective inductive reaction of the grid circuit of a tube in the plate circuit of the preceding tube together with the inductive reaction of the plate circuit itself will always be below the critical value required to cause the preceding tube to oscillate, and meansfor preventing-undesired magnetic coupling between the transformers.

Signed at East Orange, N. J., this first (lay' of March, 1924.

MILLARD COLE SPENCER. 

